In the modern semiconductor industry the reduction of the layer thickness of a substrate is an essential prerequisite for the continuously advancing miniaturization of the semiconductor components. A few years ago, active and passive components were produced only on a single substrate, especially a wafer. Today, substrates of different functionality are aligned to one another and bonded to one another. Contact is made between the individual function units, preferably through silicon vias (TSVs). The TSVs allow direct production of an electrically conductive connection between the substrates.
One of the critical processes in the production of such a multisubstrate stack is the back-thinning process. In a back-thinning process a substrate with a diameter of generally several hundred millimeters is thinned to a layer thickness of down to less than 50 μm. The substrate is therefore mechanically stabilized during the back-thinning process using a carrier substrate. Moreover the stabilization technology is also intended to allow further processing, transport, and storage of the few micron-thick substrate.
The most frequently used carrier technology is so-called temporary bonding. In temporary bonding a temporary cement is applied to the product wafer (which is to be back-thinned) and/or the carrier substrate. Afterwards a temporary bonding process between the product substrate and the carrier substrate takes place. Here, it is often such that the product substrate side, which is fixed with the temporary cement on the carrier wafer, already has functional units. The functional units can be microchips, LEDs, MEMs, RDLs, or bumps. The layer thickness of the temporary cement must be accordingly greater than the maximum height of the functional units in order to ensure complete embedding of the functional units in the cement.
A very serious problem in the prior art is the so-called total thickness variation (TTV). This is defined as the difference between the maximum and minimum thickness of an object along one given direction or one plane. For example, the TTV of a silicon wafer is defined as the difference between the maximum thickness which is to be found at one location of the silicon wafer and its minimum thickness which is to be found elsewhere.
The concept of the TTV can also be expanded to combined objects. Thus, in the semiconductor industry the thickness variation of a bonded substrate stack is generally measured. In doing so the substrate stack with the surface of its first substrate pointing to the outside is deposited on a plane of a measuring device which is as planar as possible. Afterwards a relative movement of the substrate stack with respect to a sensor takes place. The sensor is generally a laser interferometer. With one laser interferometer thickness changes into the nanometer range can be detected and ascertained. A measurement of the substrate stack thickness along the measurement direction takes place by the relative movement between the bonded substrate stack and the laser interferometer. Layer thickness maps and the maximum and minimum layer thickness, and thus the TTV, can be determined from the profile which has been obtained in this way.
The knowledge of the TTV is important mainly for those processes which are applied to the product substrate after bonding, mainly back-thinning and etching processes. An overly large TTV provides specifically for nonuniform thickness removal and thus leads to destruction of the product substrate in the worst case.